A. Field of the Invention
The invention relates to an improvement in the process for preparing fiber-forming, random copolymers comprising recurring units of the formula ##STR1## IN A MOLE RATIO OF 30:70 TO 80:20 BY THE MELT POLYMERIZATION OF A MIXTURE OF MONOMERS COMPRISING AT LEAST 85 MOLE % HEXAMETHYLENEDIAMMONIUM TEREPHTHALATE (6TA salt) and hexamethylenediammonium isophthalate (6IA salt), whereby the amount of ammonia formed during polymerization is reduced.
As used herein the term 6TA/6IA copolymer refers to the above copolymer consisting essentially of 6TA/6IA units.
B. Description of the Prior Art
It is generally known in the art that 6TA/6IA copolymers may be prepared in a conventional manner by the melt polymerization of a mixture of 6TA and 6IA salts such as by the procedures described in British Pat. No. 1,382,551, U.S. Pat. No. 3,941,755. It is a common practice to use as the starting material for the melt polymerization an aqueous solution of the 6TA and 6IA salts. The salts are made by mixing stoichiometric amounts of hexamethyleendiamine (HMD), terephthalic acid (TA) and isophthalic acid (IA) in water. In principle, the conversion of the salts to 6TA/6IA copolymer involves heating the salts to a temperature at which condensation of --NH.sub.2 and --COOH groups occurs, releasing water and forming ##STR2## linkages. In practice, the aqueous salt solution is first concentrated to, for example, 60% and then charged to an autoclave. After purging air from the autoclave the contents of the autoclave are heated to about 220.degree. C. while the pressure in the autoclave is permitted to rise slowly until it reaches about 250 psig, at which point steam is slowly bled off so as to maintain this pressure. The contents are then heated to about 300.degree. C. or higher and held at this temperature for a period of time while holding the pressure substantially constant at about 250 psig by bleeding off steam. Then, the temperature is increased slightly, for example 10.degree. to 30.degree. C., while the pressure is reduced to atmospheric pressure. The resulting molten mass may be held at atmospheric pressure at or above the melting temperature thereof (e.g. 325.degree. C.) for a period of time sufficient to permit escape of entrained water vapor, for example, 30 to 60 minutes. The optimum time, temperature and pressures involved in the melt polymerization will vary somewhat depending on the melting point of the particular 6TA/6IA copolymer being prepared.
It has been observed that significantly larger amounts of ammonia are formed during the melt polymerization of the 6TA/6IA salt mixture than are formed during the melt polymerization of, for example, polyhexamethylene adipamide. It is believed that the reason for this is due at least in part to the higher temperatures (e.g. &gt; 300.degree. C.) employed during the melt polymerization of the 6TA/6IA salt mixture. The safe disposal of by-products, such as ammonia, while being desirable from the standpoint of environmental considerations, adds to the overall cost of preparing 6TA/6IA copolymers.
Accordingly, an object of the present invention is to provide an improvement in the melt polymerization process for preparing 6TA/6IA copolymers, whereby the amount of ammonia formed during polymerization is reduced.